AOA application of digital channelized IFM receiver

Abstract

This system compares the phases from Fast Frequency Transform (FFT) outputs to obtain angle of arrival (AOA) information.

What is claimed is: 
     
       1. Apparatus for measuring an angle of arrival AOA of an incoming signal using at least two antennas designated as first and second antennas separated by a distance d; wherein said apparatus uses a digital channelized IFM (Instantaneous Frequency Measurement) receiver comprising first means for receiving RF input signals from at least one of said antennas and for converting the RF input signals to IF signals, analog-to-digital conversion means coupled to the first means for converting the IF signals to digital signals at D data points, the analog-to-digital conversion means being coupled to digital filter bank means for providing channelized digital signals using a P point short FFT (Fast Fourier transform) to perform the channelization to provide i=1 to P/2 channels, digital IFM receiver means for processing the channelized digital signals to effectively provide digital IFM receiver means for the output of each channel;   wherein the short FFT is overlapped P-1 points, and the output of each channel is written as I i  (t j ) and Q i  (t j ) where I and Q are inphase and quadrature components of a complex form, i represents output channel number, and j=1 to (D-P+1) represents the output time, and the means for finding a phase Θ i  of each output signal as ##EQU6## means for finding the phase difference of each channel as   δΘ.sub.i (t.sub.j)=Θ.sub.i (t.sub.j+1)-Θ.sub.i (t.sub.j)     and means for finding the frequency of the output signal from ##EQU7## wherein the frequencies of all channels are measured, if the frequency measured by an IFM receiver matches the center of a channel, that channel contains a signal, and if the frequency measured by the IFM receiver does not match the center of the channel, that channel does not contain a signal, so that using this measurement, the channels containing signals can be identified;     wherein said analog-to-digital conversion means includes first and second converters coupled respectively to said first and second antennas to provide first and second sets of digitized samples in channels i at times j, first and second Fast Fourier Transform (FFT) means coupled respectively to outputs from the first and second converters which perform Fast Fourier Transforms on the digitized data;   means coupled to the outputs of the first and second Fast Fourier Transform (FFT) means for processing the data from the last said outputs using the relation ##EQU8## where Θ ab ,i{t.sbsb.j }   is the phase difference between the two antennas, f is the frequency of the input signal, X is a multiplication symbol, and C is the speed of light, the separation d between the first and second antennas being less than half the wavelength of the input signal to avoid ambiguity.   
     
     
       2. Apparatus as set forth in claim 1, wherein in response to there being simultaneous input signals of different frequencies, the phase difference measured at different frequency channels, i.e. different i values, is used to find the angle-of-arrival AOA of each of them. 
     
     
       3. Apparatus as set forth in claim 1, wherein the phase differences at many different times i.e. different j values, are used to find the AOA, averaged over time to improve the measurement accuracy.